On the consequences of the uncertainty principle on the superconducting fluctuations well inside the normal state

Laboratorio de Baixas Temperaturas e Superconductividade Departamento de Física da Materia Condensada Universidade de Santiago de Compostela - Santiago de Compostela, E-15782, Spain

Received: 15 April 2002
Accepted: 5 June 2002

Abstract

We first argue that the collective behaviour of the Cooper pairs created by thermal fluctuations well above the superconducting transition temperature, T_c, is dominated by the uncertainty principle which, in particular, leads to a well-defined temperature, T^c, above which the superconducting coherence vanishes. On the grounds of the BCS approach, the corresponding reduced temperature, , is estimated to be around 0.55, i.e., above coherent Cooper pairs cannot exist. The implications of these proposals on the superfluid density are then examined using the Gaussian-Ginzburg-Landau approximation. Then we present new measurements of the thermal fluctuation effects on the electrical conductivity and on the magnetization in different low- and high-T_c superconductors with different dopings which are in excellent agreement with these proposals and that demonstrate the universality of .